1. Field of the Invention
The present invention relates generally to hydraulic control systems and more particularly to an all-hydraulic horizontal stabilizer trim control surface position control system for use on a fixed-wing aircraft.
2. Prior Art
Fixed-wing aircraft horizontal stabilizer control surfaces typically require position trimming by the pilot to respond to certain aircraft flight conditions such as takeoff, in-flight and landing. Various means of accomplishing horizontal stabilizer trim surface control have been utilized on a variety of fixed-wing aircraft such as electromechanical, electrohydraulic and mechanical trim control systems. Adjusting the angular position of the horizontal stabilizer trim control surface aids in the pitch control of the aircraft.
In one type of fixed-wing aircraft, the movement of the horizontal stabilizer, located adjacent to the vertical stabilizer and hinged at the rear spar to permit up and down trim movement of the leading edge, may be provided by an electrical trim control system. Means for indicating the travel/position of the horizontal stabilizer in degrees may also be included. The common modes of electrical trim control operation are autopilot trim and manual electrical trim. The autopilot trim allows automatic trimming of the horizontal stabilizer when the autopilot system is engaged. The manual electrical trim provides pilot control of the horizontal stabilizer trim by means of a cockpit toggle trim switch on the control wheel. Actuation of the switch disengages the automatic flight control system and allows the pilot to manually adjust the angular position of the horizontal stabilizer trim control surface. In this case, placing the pilot""s toggle switch in the nose-up position usually energizes a nose-up relay and drives the trim actuator (via the actuator control unit) to the desired nose-up position. Nose-down trim is achieved in a similar manner using a nose-down relay. The trim actuator generally includes an electric motor, brake, clutch, reduction gear train, mechanical input shaft, limit switches and a position transmitter. The electrical motor drives the reduction gear train via the clutch. When the motor is turned on, the brake releases the motor shaft. The mechanical input shaft provides a means of driving the actuator gear train from an external source. The position transmitter (e.g., potentiometer) provides position information to the actuator control unit which amplifies the signal and transmits the same to the horizontal stabilizer trim position indicator in the cockpit.
Electro-hydraulic trim control systems may be intermittent duty or continuous duty systems in which some hydraulic fluid pressure is always applied to the trim actuator attached to the control surface. Control surface positional changes are made by altering the pressure differential applied to the associated actuator. Many of these control systems include some type of feedback arrangement to allow precision surface position control.
A trim control system of this type is shown, for example, in U.S. Pat. No. 4,840,031 to Hribar which deals with a control system for an actuator used to position a control surface on an aircraft such as a horizontal stabilizer. The control system includes a source of pressurized hydraulic fluid, a pressure control for establishing a predetermined pressure level that is substantially one-half source pressure, a blocker valve and a direction control valve for controlling the application of fluid pressure to the actuator. When idle, the blocker valve supplies substantially equal control pressures to the actuator so that the control system remains pressurized to substantially one-half source pressure. When the actuator is being energized, the direction control valve determines the direction of actuation and the blocker valve controls the fluid flow rate to and from the actuator to maintain a constant actuation rate regardless of load. A servo mechanism monitors return flow across an orifice and adjusts the position of a blocker valve element in order to throttle both the input and return fluid flows to the actuator as a function of the return fluid flow rate.
The above-described trim control systems are fairly complex systems which commonly provide performance at the expense of reliability. Reliability of control systems of this type and especially of horizontal stabilizer trim control systems has been an issue of growing concern for quite some time for aircraft manufacturers and pilots alike. In particular, failures and/or near-failures of horizontal stabilizer trim control systems have been occurring lately at an alarming rate.
Therefore, the need arises for a reliable, preferably all-hydraulic horizontal stabilizer trim control system which may be utilized in a variety of fixed-wing aircraft. A control system of this kind may include a primary hydraulic motor, a backup electrical motor, hydraulic valves and a gear train for proportional adjustment of the angular position of the horizontal stabilizer control surface and for coupling the two motors. One of the valves may be a spool-and-sleeve rate control valve for proportionally controlling the flow rate to the hydraulic motor ports based on actual control surface position. Proportional flow rate control may be achieved by mechanical feedback means in the form of a linkage between the horizontal stabilizer control surface and the spool of the rate control valve. In case of mechanical feedback failure, the rate control valve spool would be automatically repositioned to allow a pre-determined default hydraulic flow rate to the motor ports of the hydraulic motor. An all-hydraulic horizontal stabilizer trim control system of this type will provide a viable solution to the above-described problems of the prior art.
The present invention is directed to a horizontal stabilizer trim control surface position control system for use by a pilot on an aircraft, comprising a trim controller for controlling the angular position of the horizontal stabilizer trim control surface on command by the pilot, the trim controller powered substantially by pressurized hydraulic fluid; a trim actuator operatively coupled between the trim controller and the horizontal stabilizer trim control surface for adjusting the angular position of the horizontal stabilizer trim control surface on command by the pilot, the trim actuator driven by the trim controller; and means for providing feedback on the angular position of the horizontal stabilizer trim control surface to the pilot.
The trim controller includes a plurality of spool-and-sleeve hydraulic valves, each of the spool-and-sleeve hydraulic valves having a plurality of inlet and outlet ports for flowing pressurized hydraulic fluid. The plurality of spool-and-sleeve hydraulic valves includes a rate control valve, a directional control valve hydraulically coupled to the rate control valve, a blocking-bypass valve hydraulically coupled to the directional control valve and a shutoff valve hydraulically coupled to the blocking-bypass valve, the rate control valve operatively coupled to on-board aircraft hydraulic system supply and return lines.
Means for driving the directional control valve is provided. The directional control valve driving means includes a first solenoid valve operatively coupled to one end of the directional control valve and a second solenoid valve operatively coupled to another end of the directional control valve, the first and second solenoid valves actuated by the pilot.
Means for actuating the first solenoid valve and the second solenoid valve by the pilot is also provided. The actuating means includes a toggle trim switch for selectively actuating the first solenoid valve and the second solenoid valve by the pilot through a pilot interface operatively coupled between the toggle trim switch and the first and second solenoid valves, the toggle trim switch and the pilot interface powered by an on-board aircraft power source.
In accordance with one aspect of the present invention, the trim actuator comprises at least one motor having a motor shaft, a first gear train driven by the motor shaft and means for adjusting the angular position of the horizontal stabilizer trim control surface on command by the pilot. The angular position adjusting means includes an output shaft operatively coupled between the horizontal stabilizer trim control surface and the first gear train, the output shaft driven by the first gear train, the driven output shaft having linear displacement, the linear displacement adjusting the angular position of the horizontal stabilizer trim control surface on command by the pilot.
The motor may be a hydraulic motor having a first motor port and a second motor port, the first and second motor ports hydraulically coupled to some of the ports of the shutoff valve, the shutoff valve controlling the flow of pressurized hydraulic fluid to the first and second motor ports. The first gear train comprises a pinion gear driven by the motor shaft of the hydraulic motor, a spur gear driven by the pinion gear and a first worm gear set driven by the spur gear.
The first worm gear set comprises a worm driven by the spur gear and a worm gear driven by the worm, the output shaft driven by the worm gear, the output shaft linear displacement resulting from the worm gear driving the output shaft.
In accordance with another aspect of the present invention, means for providing feedback on the angular position of the horizontal stabilizer trim control surface to the rate control valve is provided. The rate control valve feedback means includes a second gear train driven by the first gear train and operatively coupled to a feedback shaft, the feedback shaft coupled to the spool of the rate control valve for linearly displacing the spool of the rate control valve inside the sleeve of the rate control valve substantially in proportion to the linear displacement of the output shaft.
The second gear train comprises a bevel gear set driven by the first gear train and a second worm gear set driven by the bevel gear set, the feedback shaft driven by the second worm gear set. The bevel gear set comprises a first bevel gear driven by the worm gear and a second bevel gear driven by the first bevel gear. The second worm gear set comprises a second worm driven by the second bevel gear and a worm gear segment operatively coupled between the second worm and the feedback shaft for driving the feedback shaft substantially in proportion to the linear displacement of the output shaft.
In accordance with yet another aspect of the present invention, means for controlling the flow rate of pressurized hydraulic fluid to the first and second motor ports of the hydraulic motor is provided. The flow rate control means includes a groove on the spool of the rate control valve for passing outflowing pressurized hydraulic fluid from the spool and at least one outlet flow slot on the sleeve of the rate control valve for accommodating the outflowing pressurized hydraulic fluid from the spool groove, the at least one sleeve outlet flow slot providing a variable pressurized hydraulic fluid outflow area for varying the pressurized hydraulic fluid flow rate to the first and second motor ports of the hydraulic motor substantially in proportion to the angular rate of displacement of the horizontal stabilizer trim control surface.
In accordance with a still another aspect of the present invention, means for providing feedback on the angular position of the horizontal stabilizer trim control surface to the shutoff valve is provided. The shutoff valve feedback means includes the second gear train driven by the first gear train and operatively coupled to the feedback shaft, the feedback shaft coupled to the spool of the shutoff valve for linearly displacing the spool of the shutoff valve inside the sleeve of the shutoff valve substantially in proportion to the linear displacement of the output shaft.
In accordance with a different aspect of the present invention, the means for providing feedback on the angular position of the horizontal stabilizer trim control surface to the pilot includes a horizontal stabilizer trim control surface position sensor operatively coupled to the spool of the shutoff valve, the horizontal stabilizer trim control surface position sensor powered by an on-board aircraft power source.
In accordance with a still different aspect of the present invention, a backup electric motor may be included for use during loss of hydraulic system supply pressure, the pilot interface automatically actuating the backup electric motor during loss of hydraulic system supply pressure, the backup electric motor powered by an on-board aircraft power source. In this case, the first gear train would include a pinion gear driven by the motor shaft of the backup electric motor, a spur gear driven by the pinion gear with the first worm gear set driven by the spur gear.
These and other aspects of the present invention will become apparent from a review of the accompanying drawings and the following detailed description of the preferred embodiments of the present invention.